1. Field of the Invention
The present invention relates to an optical passive device such as a polarization-independent optical isolator, a polarization-independent optical circulator, and the like used in an optical fiber amplifier and the like, and an optical fiber amplifier using these devices.
2. Description of the Related Art
In recent years, with the practical use of an optical fiber amplifier, demand for a polarization-independent optical isolator for preventing deterioration of its S/N ratio caused by a reflected returning light has been increased.
A polarization-independent optical isolator is composed of a birefractive material such as rutile and the like, a Faraday rotator such as YIG and the like. A polarization-independent optical isolator separates an optical path into a forward path and a backward path which are different from each other, as using an optical non-reciprocity of a Faraday rotator. This optical isolator is required that it has a low insertion loss to a forward light and a high insertion loss to a backward light.
Many of polarization-independent optical isolators currently in practical use are of a type called an inline type in which an input and an output optical fiber are opposite to each other by 180 degrees in relation to their forward direction. In an existing polarization-independent optical isolator, an input and an output optical fiber are opposite to each other by 180 degrees in relation to their forward direction. Since an optical fiber cannot be bent usually with a radius of 30 mm or less, it is necessary to have a margin for a bending radius (about 30 mm) of the fiber at both sides of the optical component. This becomes an obstacle to miniaturization of an optical fiber amplifier in implementation.
For the purpose of miniaturization of an optical fiber amplifier, an optical component in which a polarization-independent optical isolator, an optical branching film, a PD device, and the like are integrated has been implemented. The integrated optical component also becomes an obstacle to miniaturization of an optical fiber amplifier since it needs to have a margin for a bending radius of the fiber at both ends of it in implementation similarly to an optical amplifier using an isolator of inline type shown in FIG. 2.
Up to now, an optical circulator having three or four ports has been proposed as an optical circulator. An existing optical circulator is composed of two polarization beam splitters, a 45-degree Faraday rotator, and an optical rotator. In an existing configuration, in case of composing an optical circulator of four ports, input/output terminals are disposed in four directions on the optical circulator body. In such a configuration as this, since a margin for a bending radius of the optical fiber is provided in the four directions in implementation, a large space is needed for implementation.
As a configuration to improve this, there is an optical circulator composed of a birefringent crystal, a 45-degree Faraday rotator, and an optical rotator. In case of composing an optical circulator of four ports according to this configuration, input/output terminals are disposed in two directions opposite to each other by 180 degrees on the optical circulator body.
In any of the above-mentioned configurations of optical passive components, since the optical input/output terminals are opposite to each other by 180 degrees, a margin for a bending radius is necessary at both sides of the optical component. This is an obstacle to miniaturization of an optical fiber amplifier in implementation.
Furthermore, in an existing optical circulator, the number of ports to be provided on an optical circulator is limited to four and it is difficult to implement an optical circulator of more ports than this.